The present disclosure generally relates to power stages and voltage converters, and especially to DC-DC converters or switched voltage regulators, capable of varying output voltage and current as a function a processing load of circuits powered by such a converter.
Switched voltage converters are used to convert between differing DC voltages in a wide range of applications. Among switched voltage converters, step-down converters are used to provide a reduced voltage from a higher voltage supply. Typical uses of switched power stages comprise DC-DC converters in particular for battery-operated devices, power stage for class-D amplifiers including audio amplifiers, motor drive circuits, photovoltaic inverters, etc. Such a switched power stage is schematically shown in FIG. 1. The power stage PWS comprises switches SW1, SW2 which are used to alternately connect a first terminal of an inductor L1 to a supply voltage IV and to a k-low voltage such as ground voltage, at a switching frequency. A second terminal of the inductor L1 is connected to a load LD and linked to the ground by a capacitor C1. The switches SW1, SW2 are controlled by respective signals SH and SL provided by a control circuit CTL, so that when the switch SW1 is turned on, the switch SW2 is turned off and conversely.
In battery-operated devices such as mobile phones, smart phones, digital tablets, there is a need to increase the battery life. To this purpose, the circuits of the device that are not used are powered off or receive a reduced power. Thus the supply current requested by the device may dramatically vary. When one or more circuits of the device are deactivated, the current drawn by the device may drop very shortly, thus resulting in a voltage overshoot if the supplied current does not follow this drop. This voltage overshoot may be reduced by increasing the size of the capacitor C1.
In addition to the voltage overshoot, the current ripple of the inductor should be taken into account to reduce switching core loss of the inductor and keep the peak current within the maximum current rating of the inductor and the battery. The optimization of switching losses while maintaining the average current loads closer to the maximum rating constrains the range of inductor values appropriate for a given input to output voltage ratio and operating frequency. For a DC-DC converter operating with 1 Mhz or slower PWM control, the inductor should typically be sized to 1 μH or larger to meet these constraints. Such a big inductor cannot be compact and integrated in a semiconductor chip. Conversely when a circuit of the powered device is activated, it should be powered on in a very short time, inducing a sudden rise of the current drawn by the device. One way to follow such a current draw is to reduce the size of the inductor L1.
Preferably, components of small height and reduced surface on printed-circuit boards are used to manufacture thin and small devices. This generally serves to reduce the size of inductor L1 and capacitor C1, and thus to increase the commutation frequency of the switches SW1, SW2, which increases the energy losses in the switches.
Further, each new generation of processors used in such portable devices tends to be more powerful while being smaller and operating at lower supply voltages. In addition, to increase their life by reducing the current supplied by each battery cell, the number of battery cells assembled both in series and in parallel within the batteries tends to increase. Accordingly the input voltage of the DC-DC converter tends to increase whereas the output voltage to be supplied to the devices tends to decrease, which requires a bigger inductor. This results in subjecting the inductor to conflicting requirements.